I tried proprietory slide masking tape and found it a) expensive b) hard to use and c) not very effective. Much better was aluminium baking foil, but rather than cut it with a scalpel for straight edges, I folded it carefully to give a clean, straight, crisp edge, securing it in the 35mm mount with the adhesive masking tape. I take no credit for this technique - it was recommended to me by Margaret Salisbury FRPS several years ago.
Best wishes,
Steve